This invention relates to frequency conversion means for producing coherent radiation at optical frequencies not easily available from present lasers, and more particularly relates to nonlinear optical means for doubling or mixing the output of one or more solid state lasers in an external cavity which is resonant at the wavelengths of the pump lasers and the generated wavelengths, in order to produce high conversion efficiency and high output power stability despite low input power.
It has been known since the early 1960s that certain nonlinear crystals have the property of converting a fraction of the input power at a given optical frequency into an output beam at twice the optical frequency. The same nonlinearity can mix two input beams to produce an output beam of the sum frequency.
A. Ashkin et al. [IEEE J. Quant. Elect. QE2 109-124 (1966)] described theoretically how to enhance the efficiency of doubling or mixing by resonating either the fundamental frequency or the generated frequency. They showed how the generated power depends on the optical loss inside the resonator, and pointed out the advantage of noncritical phase matching. In the case of fundamental resonance, they describe how to optimize the input coupling transmission, including the effect of mode conversion loss, as was known at the time for microwave resonators. They successfully demonstrated both doubling and mixing of a two-mode helium-neon laser with harmonic resonance only, and doubling with fundamental resonance only. They decided against using dual resonance (simultaneous resonance for both the fundamental and the harmonic), because of phasing problems on multiple passes through the crystal.
This pioneering work led to the development of singly resonant devices which used the resonant cavity of a laser to enhance the second harmonic generation. There is a large body of literature describing nonlinear conversion inside the cavity of a laser, where the laser frequency is either doubled, mixed with the output from another laser, or even mixed with the residual light from the pump laser which excites the first laser. None of this prior art is directly relevant to the invention. As explained hereinafter the present invention uses an external resonant cavity.
M. Brieger et al. [Opt. Commun. 38 423-426 (1981)] were the first to show that improved conversion efficiency can be obtained by resonating the pump laser in a cavity external from the laser resonator. Their work has been followed up by many other papers reporting similar work with singly resonant external cavities, all of which used noncritically phase matched nonlinear crystals. This work includes generation of ultraviolet light from dye and Argon lasers [H. Hemmati et al., Opt. Lett. 8 73-75 (1983)], the generation of blue light from dye lasers [J. C. Baumert et al., Appl. Opt. 24 1299-1301 (1985)], and the generation of green light from YAG lasers [W. J. Kozlovsky et al., U.S. Pat. No. 5,027,361; W. J. Kozlovsky et al., IEEE J. Quant. Elect. QE24 913-919 (1988)].
Diode lasers have been used to generate blue light only in singly resonant noncritically phase matched KNbO.sub.3 [L. Goldberg et al., Appl. Phys. Lett. 55 218-220 (1989); W. J. Kozlovsky et al., Appl. Phys. Lett. 56 2291-2292 (1990); A. Hemmerich et al., U.S. Pat. No. 5,068,546; and A. Hemmerich et al., Opt. Lett. 15 372-374 (1990)].
After more than ten years of development, this prior art can be considered to be reasonably mature. It is sufficient for producing a good conversion efficiency (1.7%-39%) in two wavelength ranges where noncritical phase matching can be used (532 and 421-432 nm) with nonlinear crystals having a high nonlinearity (LiNbO.sub.3 and KNbO.sub.3, respectively).
Nevertheless, the prior art suffers from several difficulties. Power fluctuations in the pump laser are amplified by the nonlinearity, producing instability in the output power at and above the 8% level observed by Kozlovsky et al. in their high efficiency doubled YAG output. In the ultraviolet, no high nonlinearity materials are available. Only high power dye or Argon lasers have been used to produce a UV output, but an input power of 2 W was needed to produce a 4% noncritically phase matched conversion efficiency to 257 nm [J. C. Bergquist et al. Opt. Commun. 43 437-442 (1982)]. Lower power lasers such as the desirable solid state lasers (including diode and diode-pumped lasers) are insufficient for producing usable output power in the UV even with noncritical phase matching, since the output power drops as the square of the input power. Most regions of the spectrum are inaccessible with solid state lasers and the current state of the art since they require critical phase matching, which lowers the conversion efficiency even further due to the effects of walkoff.
Ashkin et al. mentioned but rejected the use of dual resonance (harmonic resonance at the same time as the pump laser resonance) for increasing the generated power. This alternative has been rejected many times over the years. A solution to the multiple pass phasing problem was devised in 1971 by J. M. Yarborough et al. [Appl. Phys. Lett. 18 70-73 (1971)] and tested in a two-pass configuration. Unfortunately, the phase alignment obtained by this means is only maintained at a specific temperature, pressure, and humidity. The phase alignment wanders off if any of these ambient conditions change. After nine more years, P. D. Drummond et al. [Optica Acta 27 321-335 (1980)] analyzed theoretically the small signal dynamics of the dual resonant doubler and predicted an output power instability above a certain pump power threshold.
The first experimental reference to a dual resonant doubler is by H. J. Kimble et al. [Quantum Optics IV. J. D. Harvey, D. F. Walls, eds., Springer-Verlag, pp. 58-69, 1986] who doubled an Argon laser with KDP. S. F. Pereira et al. [Phys. Rev. A38 4931-4934 (1988)] later built a dual resonant doubler for producing green light from a YAG laser With MgO:LiNbO.sub.3. Both of these experiments were performed with noncritically phase matched doubling crystals in linear concave-concave cavities. Neither report describes how to obtain dual resonance in the presence of walkoff (critical phase matching). G. J. Dixon et al. [U.S. Pat. No. 4,884,276] mentioned the possibility of a dual resonant optical mixer in the context of noncritical phase matching, but again did not describe how to deal with cavity stability in the case of critical phase matching.
Finally in 1989 a critically phase matched dual resonant doubler was reported in the literature [C. Zimmermann et al., Opt. Commun. 71 229-234 (1989)]. In this work, an argon laser beam at 488 nm was doubled at 0.4% conversion efficiency in barium borate phase matched at an angle of 55 degrees. Heretofore there has been no subsequent work published respecting this approach.
The dual resonant doubler as described in the prior art suffers from several important disadvantages. Zimmermann et al. state that critically phase matched dual resonance requires a linear plano-concave cavity geometry to produce a closed round trip for both ordinary and extraordinary beams. This result rules out the desirable ring resonator, the concave-concave linear approach used in the prior noncritically phase matched work of Kimble, Pereira, et al., and many other cavity configurations.
Zimmermann et al. also describe two ways of obtaining simultaneous resonance at the fundamental and second harmonic frequencies: adjusting the overall length of the cavity using the dispersion of air as described by Yarborough et al., and adjusting the phase matching angle of the nonlinear crystal. These approaches each have unresolved difficulties. Adjusting the cavity length in air to achieve dual resonance has the problem that changes in the environment shift the phase alignment away from optimum. For example, a temperature change of 13.degree. C. in a 20 cm air path is enough to shift phase by about 90 degrees in a 700 nm doubler. Compensating for such changes in real time by length adjusting is difficult because the large motions required (a few mm) can result in serious perturbations of the cavity closed path due to undesired mechanical coupling or off-axis reflection. Also, the required displacement is too large for the most convenient actuators (such as PZTs) needed for automatic phase adjustments. Changing the phase matching angle to compensate the phase drift in air reduces the conversion efficiency (by 36% in the above example).
A doubling approach is needed which permits high enough conversion efficiency for diode and diode-pumped solid state lasers to be used despite their relatively low power. Improvements are needed which stabilize the output power fluctuations and which permit generation at more wavelengths than are now possible. Improvements are also needed which reduce the numerous sensitivities of the prior art to drifts.
My U.S. Pat. No. 5,134,622 filed Dec. 20, 1990 sets forth as one example a dual resonant mixer with two laser pump sources. Claim 18 recites such a configuration. My co-pending parent patent application Ser. No. 07/826,316 filed Jan. 24, 1992, now U.S. Pat. No. 5,206,868, describes a dual resonant mixer configuration which includes doubling a single laser pump beam. Claims 22 and 31 recite such configurations. Neither of my other related patent properties provide details about certain solutions to the problem of walkoff.
What is needed are improvements, in particular in procedures for resonator design, which optimize overlap of the pump beam with the acceptance volume in phase space for generating the output beam, allowing the conversion efficiency to be significantly enhanced.